1. Field of Invention
The invention relates to a vehicle driving unit which employs a driving source formed by combining a combustion engine (hereinafter referred to as an engine) with a motor generator and a multi-stage automatic transmission for accomplishing a plurality of speeds. More specifically, the invention relates to a control system of the vehicle driving unit.
2. Description of Related Art
Generally employed vehicle driving units include a hydraulic driving unit which combines a driving source formed of an engine and a motor generator with a multi-stage automatic transmission. The motor generator of the driving unit of this type can be used as a generator which recovers braking energy from wheels and accumulates it as electric power. The motor generator is driven by the accumulated power to start the engine and drive the vehicle.
As previously designed, the aforementioned hybrid vehicle driving unit may use a torque converter as a starting device which inputs a driving torque of the driving source formed by the engine and the motor generator into the automatic transmission. Alternatively, a power split device formed by combining a planetary gear with a clutch can be used in the driving unit of the aforementioned type.
Unlike the torque converter, the power split device does not retard revolution change resulting from fluid slippage when the automatic transmission performs a shift operation while the engine is being driven. As the input revolution changes, the inertia torque generated by an inertia moment at the front side of the transmission such as an engine and a fly wheel during the shift operation is directly input to the transmission. On the other hand, the input inertia torque is rapidly absorbed by the engaging elements at the end of the shift operation. Such a sharp change in the torque may cause a great shift shock.
However, a shift shock resulting from rapid absorption of the inertia torque, though on a relatively small level, occurs also in a generally employed multi-stage automatic transmission provided with a general torque converter having only an engine as a driving source. In order to eliminate the aforementioned shift shock, the generally employed automatic transmission is provided with various kinds of devices for controlling an engagement hydraulic pressure applied to the friction engagement elements. In case of following the general solution as aforementioned to eliminate the shift shock which occurs in the hybrid vehicle driving unit, the engagement pressure applied to the friction engagement elements is decreased immediately after the end of the shift operation to reduce a rate of the revolution change. This may prevent the output torque from sharply declining.
FIG. 7 is a timing chart showing the relationship between characteristics of an engagement hydraulic pressure (Pa) of the friction engagement elements, an engine revolution (Ne) and a transmission output torque (Tout). It is assumed that the engagement hydraulic pressure (Pa) of a clutch or a brake as the friction engagement element decreases from the level of the general characteristic shown by a chain line to the level shown by a solid line at a later stage of the engagement phase as the arrow indicates. Then, a sharp decline of the transmission output torque (Tout) resulting from absorption of the inertia torque shown by a chain line can be modified to a gentle decline shown by a solid line.
In the case where the aforementioned method is adopted to inhibit the shift shock resulting from absorption of the inertia torque, the transmission output torque (Tout) characteristic may cause the time required for the shift operation to be longer by (t) than the case where generation of the inertia torque is not retarded as shown by the chain line. Accordingly, the time period during which the friction engagement elements slip is extended by the time (t). This may adversely affect the durability of the friction materials constituting the respective elements.